A set of monoclonal antibodies prepared has been used in biochemical and functional analysis of rabbit lymphoid cell subpopulations. Detailed study has defined a small group of the monoclonal antibodies which recognize distinct cell surface molecules and define among them subsets of T cells in the thymus and periphery as well as subsets of B cells. Immunoprecipitation of labeled surface molecules has defined the antigens while the cell sorter has been used for cellular analysis. A number of antigen-specific rabbit T cell lines have been developed to characterize rabbit T cell responses to the group C streptococcus. These lines are being propagated for studies on antigen-binding molecules and restriction in antigen presentation. The kappa chain allotypes of wild rabbits (b97,b98) are under structural study in collaborative experiments. The hamster serum protein FP (female protein) has also been partially characterized in sequence studies.